Documents often have adherent transparent overlays to provide protection against dirt, moisture, and general wear and tear. A typical protective transparent overlay has a plastic film bearing an aggressive adhesive layer by which it can be permanently adhered to the face of a document. A transparent overlay which does not obscure underlying information is disclosed in U.S. Pat. No. 3,801,183 (Sevelin et al.). The patent discloses legend-containing sheet materials which have retroreflective legend and retroreflective background areas that are substantially indistinguishable under ordinary diffuse light viewing conditions but are clearly distinguishable under retroreflective viewing conditions. Attempts to tamper with information over which the overlay has been applied result in readily detectable disruption of the overlay.
Subsequent to U.S. Pat. No. 3,801,183, a number of patents have issued disclosing other transparent overlays, each of which can be imaged with a pattern or legend that is readily noticeable only when viewed retroreflectively and can be adhesively bonded to a document without obscuring the face of the document. See, for example, U.S. Pat. No. 4,099,838 (Cook et al.) which discloses an overlay wherein the retroreflection from the background and image areas of the pattern is of contrasting colors. See also U.S. Pat. Nos. 4,688,894 (Hockert) and No. 4,691,993 (Porter et al.), which disclose transparent overlays that function similarly as those of U.S. Pat. No. 3,801,183 while having the added capability of permitting an authenticating message to be formed in the overlay after it has been adhesively bonded to a document.
The transparent protective overlays of each of the above-cited patents each comprise a monolayer of glass microspheres that makes the overlay expensive and difficult to counterfeit.
U.S. Pat. No. 4,869,946 (Clay) discloses a security card comprising a transparent upper layer having narrow parallel lenses on its outer surface and an image-containing substrate on its inner surface. The two layers form a lenticular system by which images on the substrate are selectively viewable depending upon the angle from which the card is viewed.
U.S. Pat. No. 3,503,315 (de Montebello) discloses a lenticular plate or sheet comprising adjacent lenslets with spherical convex surfaces on both sides. The sheet is used for the production and display of a picture in panoramic stereoscopic relief.
U.S. Pat. No. 4,576,850 (Martens) concerns shaped plastic articles made by replicating a microstructure-bearing surface with an ultraviolet-curable organic oligomeric composition.
A number of other patents also concern shaped plastic articles made by replicating a microstructure-bearing surface. For example, see U.S. Pat. Nos. 3,689,346 (Rowland); 4,414,316 (Conley); 4,420,502 (Conley); and 4,374,077 (Kerfeld).